The present invention relates to the use of an austenitic Cr--Ni--Mo-containing construction material for the manufacture of a composite tube with good resistance to thermal fatigue and stress corrosion when being used as bottom tubes for soda recovery units.
It has been found that the usage of such external tube material in a jointly extruded composite tube is especially advantageous when the inner tube is made of conventional carbon steel. A composite tube in this context means a compound tube of two tubular layers with a metallurgical bond therebetween. Such compound tubes are manufactured by co-extrusion or joint extrusion. Bottom tubes refer to tubes in the bottom of the soda recovery unit wherein feed water is led into the wall tubes (see FIG. 1). These bottom tubes are those tubes or parts of tubes which are disposed below the melt openings of the said soda recovery unit.
In conventional soda recovery units, the bottom tubes comprises either composite tubes of SS2333 corresponds to UNSS30409, ASTM A-213, grade Tp304H) or SS2352 corresponds to UNS S30403, ASTM A-213, grade Tp304L) as the external tube material and carbon steel as the internal tube material, or naked carbon steel tubes covered with a refractory layer. There is a disadvantage with such products since in most cases they suffer from crack formation in the outer tube component of SS2333 or SS2352 material after certain periods of service. The reason for this crack formation is both thermal fatigue due to melt breakage through the material and stress corrosion. There is no established unity of opinion in the industry about which of these factors has the dominating negative influence.
The present invention is based on the discovery that new material combinations in such composite tube applications could satisfy those requirements for construction materials used in bottom tubes in soda recovery units. The requirements to be satisfied are good thermal fatigue resistance and good stress corrosion resistance (in both hydroxide- and chloride-induced environments).